A communication network is a collection of nodes connected together to enable communication, for example data communication, between the nodes of the communication network. A node may be a computer or any other processing device or circuitry arranged to send and receive data over the communication network. Communication networks are, for example, used in vehicles. Fieldbus systems, such as, for example, controller-area network (CAN) bus, FlexRay or Local Interconnect Network (LIN) systems are used, for example, for real-time distributed control for enabling data exchange between processing devices, sensors and actuators of the vehicle. Each node contains a communication network terminal for connecting to a signal channel of the communication network. A signal channel may, for example, be a serial bus or part of a bus comprising more than one signal channel. A signal channel may refer to a signal line or signal wire. It may also refer to a wireless connection between at least two communication network terminals.
For example, LIN is a broadcast serial network comprising one master and many slaves, wherein all messages are initiated by the master with not more than one slave replying for a given message identifier. The master and slave functionalities may, for example, be implemented in microcontrollers or application-specific integrated circuits. Each master and slave contains a signalling circuit for transmitting signals on the signal channel. A signalling circuit of a LIN node may, for example, be part of a transceiver arranged to transmit signals on and receive signals from the signal channel.
As shown in FIG. 1, an example of a prior art signalling circuit 10, for example, of a LIN bus system of a vehicle, such as a car, may comprise a communication network terminal 12 connected to a voltage supply 14, such as a battery of a car, via a pull-up resistor 16 and a diode 18. For a positive supply voltage, such as, for example, +12V, the communication network terminal 12 is on a high voltage level as long as it is not connected to allow a current flow to ground 22, 24. The diode 18 may be used for protection of the voltage supply 14 from reverse current flowing into the supply due to a higher voltage on the communication network terminal 12 than on the supply 14.
A driver transistor device 20, for example, a field-effect transistor (FET), is used for connecting the communication network terminal 12 to ground 22, 24, depending on a driver control signal received at the control terminal of the driver transistor device 20, i.e. the gate of the shown FET.
A signal may be a change of a physical quantity carrying information, for example a voltage level.
The driver control signal is determined by the information to be transmitted on the signal channel. It may be a binary information, either “0” or “1”, wherein, for example, LIN is based on transmitting data through a binary model of dominant bits and recessive bits, wherein dominant corresponds to a logical 0 and recessive corresponds to a logical 1. If one node transmits a dominant bit and another transmits a recessive bit then the dominant bit is evaluated. As shown, the communication network terminal 12 may be in recessive state, i.e. on a high voltage level, until it is connected to ground.
The information to be transmitted, i.e., the transmit signal, is generated by either applying current to or drawing current from the control terminal of the driver transistor device 20, illustrated by either connecting it to a first current source 26 when closing the first switch 28 for applying current Idominant and generating a transmit signal tx=0 or connecting it to a second current source 30 when opening the first switch 28 and closing the second switch 48 for drawing current Irecessive to ground 24 and generating a transmit signal tx=1.
As shown, the driver control terminal receives a control signal that is not identical to the generated transmit signal. The gate or driver control terminal of the driver transistor device 20 may be directly connected to a capacitor 34 connected to the drain terminal of the driver transistor device 20. Alternatively, as shown, the driver control terminal of the driver transistor device 20 may be connected to a source of a source follower transistor device 32, i.e., a common drain FET amplifier, wherein the gate of the source follower transistor device 32 may be connected to the capacitor 34, which is connected to the drain terminal of the shown driver transistor device 20. This implements a feedback loop that introduces a slew rate for state change of the signal delivered at the communication network terminal 12, wherein the slew rate depends on the capacity of the capacitive device 34. The capacitor 34 may be directly connected to the drain terminal of the shown driver FET device 20 or, for example, via an isolation diode 36 for avoiding current flow into the communication network terminal 12 in case of any disturbances.
FIG. 2 schematically shows three example diagrams of voltage (in volts) vs. time (in microseconds). The transient response of a transmit signal tx 40, encountered at node 38, illustrates signalling a recessive bit, i.e., high voltage level, followed by a dominant bit, i.e., low or 0 voltage level, and by a recessive bit, i.e., high voltage level. The signal 42 encountered at the communication network terminal 12 provides the same information, with a slew rate of about 9 microseconds for a transition between a dominant and a recessive, i.e., high, signal state. The third signal 44 received at another node of the communication network or at a receiver side of a transceiver that comprises the transmitting signalling circuit 10 is a recovered version of the transmit signal, delayed at least by the time of the slew rate for signal state change in the transmitted signal.